Chemical recovery unit



Feb. 21, 1967 E. c. LEWIS ETAL 3,304,918

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E. C. LEWIS ETAL 3,304,9l8

CHEMICAL RECOVERY UNIT Filed Dec. 29, 1965 2 Sheets-Sheet 2 UnitedStates Patent OfiFiCe 3 304318 Patented Feb. 21, 1967 3,304,918 CHEMICALRECOVERY UNIT Everett C. Lewis, Avon, and Robert G. Tallent, EastGranby, Conn., assignors to Combustion Engineering, Inc., Windsor,Conn., a corporation of Delaware Filed Dec. 29, 1965, Ser. No. 517,299 2Claims. (Cl. 122-7) are lined with steam generating tubes, and thisinvention is concerned with 'an improved recovery unit Operating at arelatively high steam pressure with specific precautions 'being taken toprevent tube deterioration in the lower-most region of the furnace.

There has, in recent years, been a 'trend in the design and use ofChemical recovery units to increase the pressure of the unit, -i.e., thepressure of the steam produced by the unit for use in various processesand apparatus within the pulping *and paper making plant. As a result ofthis increase in pressure, the metal temperature of the steam generatingtubes that line the furnace of the Chemical recovery unit has,accordingly, increased. It =has been found that at steam pressures atapproximately 900 pounds per square inch and above there develops aserious problem with regard to metal wastage of the steam generatingtubes in the lower region of the furnace with this problem beingparticularly severe in the region below the primary air ports. Thepresent invention overcomes this problem by providing, in a particularmanner, a protective layer over the portion of the tubes in this regionexposed to the furnace interior.

In the operation of these Chemical recovery units the black liquor issprayed into the furnace at a location spaced well above the bottom ofthe furnace. This liquor has a substantial moisture content and most ofthis moisture is driven from the liquor spray upon its introduction intothe furnace because 'of the high temperature in the furnace and the =hotgases passing upwardly through the furnace and the spray. The solidsthat remain after thus removing the moisture fall onto the bottom orhearth of the furnace and form a roughly truncate d pile. During thedescent to the hearth some of the lighter volatiles are driven fromthese solid particles and the remaining volatiles are liberated and thecombustible material in the solids burned in this pile with thiscombustion being supported by the introduction of primary air atlocations spaced somewhat above the bottom of the furnace. This air isintroduced through ports such that it 'is directed generally Over andupon the pile of material. Some burnables are carried up through thefurnace and are consumed above the location of liquor introduction withthe 'Secondary air being introduced for this purpose.

The most severe Operating conditions insofar as deterioration of thetube metal that lines the furnace walls is encountered occur at thelocation immediately below the primary air introduction ports. A highrate Ofheat absorption is present at this location and, accordingly, the

tube metal temperatures on the furnace walls tend to be higher at thislocation than at other locations. Furthermore, there is at this locationan interface or juncture between the burning material disposed on thefurnace floor and the side walls of the furnace. Tube metaldeterioration is also experienced at a location on the walls of thechemical recovery unit intermediate the Secondary air ports and theprimary air ports. Although the metal wastage at this location is not assevere as that below the primary air ports, it is :of concern and doesrequire attention and necessitates preventive measures being taken toprevent or prohibit this metal wastage.

In order to provide a satisfactory and economic solution to this problemof tube metal wastage suitable metallic members :are 'welded to thesteam generating tubes that line the side walls of the furnace for thepurpose of securing to these tubes a layer of solidified moltenchemicals. During the operation of the Chemical recovery unit moltenchemicals are collected :at the 'bottom of the furnace and areperiodically withdrawn through a suitable spout. Molten chemicals *alsorun down the furnace walls. The furnace wall tubes are at a temperaturebelow the melting point of this molten Chemical and, accordingly, alayer of Chemical solidifies in engagement with the wall tubes. However,the tubes are normally rel-atively smooth and this layer of Chemicaldoes not adhere very well so that an un-broken layer of solidifiedchemical is not in engagement with the tubes. In a preferred embodimentof the invention the metallic means that is welded to the tubes tomaintain a solidified layer of Chemical against the tubes is in the formof expanded metal. This is advantageous in that it is very eifective tomaintain the solidified layer of Chemical intact and provide aoontinuous layer about the furnace walls and, moreover, it is relativelyeconomic. Whatever the metallic means employed to maintain this layer ofsolidified chemical in engagement with the tubes, it is necessary thatit be fabricated of Stainless steel in order to withstand the severeOperating conditions to which it is subjected.

Accordingly, it is an object of the present invention t-o provide animproved Chemical recovery unit :Operating at a steam pressure ofapproximately 900 pounds per square inch or above.

A further :object of the invention is to provide such an improvedChemical recovery unit wherein specific precautions are taken to preventtube metal wastage of the steam generat-ing tubes.

Other and further objects of the invention will become apparent to thoseskilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination of the elements of theinventive organization -in such a manner as to attain the resultsdesired, as hereinafter more particularly set forth in the followingdetailed description of an illustrative embodiment, said embodimentbeing shown by the accompanying drawings wherein:

FIG. 1 is a sectional view of a Chemical recovery unit embodying thepresent invention;

FIGS. 2 :and 3 are detailed sectional views of the lower region of thechemical recovery furnace indicating particularly the regions whereinmetal wastage of steam generat-ing tubes is a problem at pressures ofapproximately 900 pounds per square inch steam pressure or above;

FIGS. 4 and 5 are detailed views showing a preferred constructionwhereby expanded metal in the form of stainless steel is welded to thesurface of the tubes directed inwardly of the furnace at the lowerregion of the furnace in order to maintain a solidified layer ofChemical in engagement with the tubes; and

FIG. 6 is in the nature of a side elevational view of the expanded metalthat forms a part of the inventon.

Referring now to the drawings, Wherein like reference characters areused throughout to designate like elements, the illustrative andpreferred embodiment of the invention as depicted therein includes aChemical recovery unit that has a furnace 10. The walls of this furnaceare lined with steam generating tubes 12 that may 'be in tangentrelation or may be in closely spaced relation with the spaceintermediate the tubes bridged by a fin. The tubes 12 that line thefurnace form part of the heat exchange surface of the chemical recoveryunit with there being additional heat exchange surface identifiedgenerally as 14 at the upper region of the unit. The tubes 12 carry amixture of steam and water at saturation temperature for the particularpressure at which the unit is operated with this mixture passingupwardly through these tubes. The illustrative steam generator isoperative to produce steam at 950 pounds per square inch pressure withthis steam being Conveyed from the header 16 to a desired point of useand with this steam being superheated to a desired value such as 900 F.

Residual liquor obtained from the kraft pulping process is introducedinto the furnace 10 through the noZZles 18. The liquor thus sprayed intothe furnace descends downwardly toward the furnace bottom passingthrough an upwardly rising stream of combustion gases such that amajority of the moisture in the liquor is immediately evaporated withthe solid particles falling downwardly through this rising combustiongas stream and forming a pile 20 on -the hearth or 'bottom 22. A portionof the =burnables are consumed during this descent through the furnacewith additional 'burnables being consumed on the pile 20 and with thenon-burnable chemicals being smelted and periodically withdrawn throughthe discharge spout 25. As a result of this operation molten chemicalsflow down the tubes 12 which line the furnace with the body or bath ofmolten chemicals being collected at the lower region of the furnace fordischarge through said spout 25.

combustion supporting air is introduced into the furnace at twolocations. The primary air is introduced through nozzles 24 spacedrelatively close to the bottom as, for example, 3 feet above the furnacebottom, while the Secondary air is introduced through the noZZles orports 26 located above the liquor introduction noZZles 18.

As previously mentioned Chemical recovery units Operating atapproximately 900 pound per square inch steam pressure or above haveencountered a serious metal wastage problem of the tubes 12 particularlyat the location below the primary air noZZles identified in FIG. 1 as24. Significant wastage is also experienced below the Secondary airintroduction ports 26. The particular location of these areas of wastageare identified in FIGS. 2 and 3 as the general areas 28 and 29. Theapparent reason for this is that With the higher pressures thesaturation temperatu-re within the tubes 12 is elevated which, in turn,Causes the tube metal temperature to rise. The region immediately belowthese ports 24 and 26 is a region of high heat absorption such that thetube metal temperature here is substantially elevated and, furtherrnore,with relation to area 28, it is at this location that the burning on theupper surface of the pile joins or Contacts the side walls of thefurnace. In any event and for whatever reason, serious tube metalwastage problems of the steam generating tubes on the upright furnacewalls in units Operating at the aforernentioned steam pressure and abovehave been experienced.

This problem is extremely serious for several reasons, one of which sthat a tube rupture in a Chemical recovery unit creates a very hazardouscondition in that mixing water with the molten smelt in the bottom ofthe unit Causes serious explosions which have created extensive propertydamage and been the cause of loss of life for many Operating personnel.Moreover, replacement of the tubes particularly in the lower region ofthe Chemical recovery units is difiicult since the lower region isliquidtight so as to contain the molten smelt and, therefore, adjacenttubes or abutting fins are welded together to form a fiuid-tight Wallsection.

To overcome this problem of metal wastage of the steam generating tubeson the upright furnace walls at the aforernentioned locations the tubesin the lower region of the furnace are provided with means which iseffective to grip and hold a layer of solidified Chemical in engagementwith the tube as well as to produce a Cooling effect With regard to thissolidified layer. In the preferred embodiment illustratively disclosed,this means takes the form of expanded metal secured to the portion ofthe tube directed inwardly of the furnace thus providing the tube withextended surface. This expanded metal is secured to this portion of thetubes throughout the lower region of the furnace so that a sheet of thisexpanded metal extends all the way around the furnace at the interiorthereof in this lower region. The expanded metal is of stainless steel,such as Type 304, and it is comprised of a series of strands 28 unitedat bonds 30, and it is formed such that the bonds 30 and the strands areset at a Sharp angle to the plane of the sheet. To illustrate this inFIG. 6 the plane of the sheet may be identified as 32 and the angle ofthe bond may be identified as 34, and the angle of the strand identifiedas 36. The purpose of this is so that portions of the strands of theexpanded metal will be spaced from the tubes so that the molten Chemicalcan flow intermediate the strand and the tube and thus when itsolidifies, it will be securely bonded in place with the expanded metalbeing embedded in the layer of solidified Chemical.

With the lower region of the Chemical recovery unit provided with thisextended surface throughout the entire lateral extent of the furnacewalls, a layer of solidified Chemical is effectively secured inengagement with the inwardly directed face or region of the tubes atthis location in the furnace. This layer of solidified chemicalsprevents contact of the tubes by the molten Chemical and shields thetubes from the deteriorating atmosphere within the furnace and therebyalleviating to a Substantial extent the tube metal wastage problemheretofore mentioned.

The expanded metal is welded to the surface of the tubes 12 by welds 40at each of the bonds of the expanded metal. This has two effects, One ofwhich is to securely fasten the expanded metal in place and the other ofwhich is to provide good heat exchange between the expanded metal andthe tube Wall. This latter is necessary and desirable in order to keepthe temperature of the expanded metal as low as possible which, in turn,tends to build up a thicker layer of solidified Chemical. Because of thenature of the expanded metal with the many strands and because of themany welds, Cooling of the expanded metal is very effective.

While we have illustrated and described a preferred embodiment of ourinvention it is to be understood that such is merely illustrative andnot restrictive and that variations and modifications may be madetherein without departing from the spirit and Scope of the inventon. Wetherefore do not wish to be limited to the precise details set forth butdesire to avail ourselves of such changes as fall within the purview ofour inventon.

What is clai-med is:

1. A Chemical recovery unit having an upright furnace into which blackresidual liquor produced from kraft pulping process is introduced forburning and smelting,

said furnace having the inner surface of its upright walls lined withsteam generating tubes forming part of a steam generatng systemoperative to generate steam of at least 900 pofinds per square inch,these walls having primary air incluction ports therein spaced somewhatabove the furnace bottom for directing air inwardly toward a pile or smelt and burnables on the bottom of the furnace, the tubes at the lowerregion of the furnace having their surface that is directed inwa-rdlyofthe furnace bare but provided with extended surface constructed andarranged to holdi a solidified layer of molten Chemical produced in thefilrnace in engagement Withthis tube surface, said extended surfacecomprises Stainless steel sheet-like means generally conforming to thetubes and welded to the tubes, said sheet-like means having numerousrelatively closely spaced openings dispersed throughout its area.

2. The Chemical recovery unit of claim 1 wherein said 5 sheet-like meanscomprises expanded metal.

References Cited by the Examiner KENNETH W. SPRAGUE, Primary Examiner.

1. A CHEMICAL RECOVERY UNIT HAVING AN UPRIGHT FURNACE INTO WHICH BLACKRESIDUAL LIQUOR PRODUCED FROM KRAFT PULPING PROCESS IS INTRODUCED FORBURNING AND SMELTING, SAID FURNACE HAVING THE INNER SURFACE OF ITSUPRIGHT WALLS LINED WITH STEAM GENERATING TUBES FORMING PART OF A STEAMGENERATING SYSTEM OPERATIVE TO GENERATE STEAM OF AT LEAST 900 POUNDS PERSQUARE INCH, THESE WALLS HAVING PRIMARY AIR INDUCTION PORTS THEREINSPACED SOMEWHAT ABOVE THE FURNACE BOTTOM FOR DIRECTING AIR INWARDLYTOWARD A PILE OR SMELT AND BURNABLES ON THE BOTTOM OF THE FURNACE, THETUBES AT THE LOWER REGION OF THE FURNANCE HAVING THEIR SURFACE THAT ISDIRECTED INWARDLY OF THE FURNACE BARE BUT PROVIDED WITH EXTENDED SURFACECONSTRUCTED AND ARRANGED TO HOLD A SOLIDIFIED LAYER OF MOLTEN CHEMICALPRODUCED IN THE FURNACE IN ENGAGEMENT WITH THIS TUBE SURFACE, SAIDEXTENDED SURFACE COMPRISES STAINLESS STEEL SHEET-LIKE MEANS GENERALLYCONFORMING TO THE TUBES AND WELDED TO THE TUBES, SAID SHEET-LIKE MEANSHAVING NUMEROUS RELATIVELY CLOSELY SPACED OPENINGS DISPERSED THROUGHOUTITS AREA.